High dynamic range (HDR) image sensors are required for many applications. Many current generation image sensors, including CCD and CMOS image sensors, have about 70 dB dynamic range. However, to match the capabilities of the human eye requires a dynamic range of up to about 100 dB. For example, an image sensor for automobile applications requires more than 100 dB dynamic range to deal with different driving conditions, such as driving through a dark tunnel into bright sunshine. Additionally, some digital still camera (DSC) sensors may need more than 90 dB dynamic range.
Many high dynamic range sensors have been proposed but each has significant drawbacks. Some of the drawbacks of previous HDR sensors include an image degradation due to fixed pattern noise, large random noise, and a reduced resolution associated with blooming.
An image sensor generates charge during an exposure time period, which is subsequently read out during a readout phase. However, a photodiode can accumulate only a certain amount of charge during an exposure time period, which limits dynamic range. One approach in the prior art to increase the dynamic range is to use a charge skimming operation to implement an exposure time control method to extend the dynamic range, which is illustrated in FIGS. 1A-1E.
FIG. 1A illustrates a conventional unit pixel 100 having a photodiode (PD), transfer transistor (TX), reset transistor (RST), and source-follower (SF) amplifier. A conventional floating diffusion (FD) node is also illustrated. The photodiode is a pinned photodiode or common n-type photodiode.
FIG. 1B illustrates the potential diagram across the unit pixel at different times (in plots 1, 2, 3, 4, 5, and 6) for the exposure control method illustrating the potential at the PD. TX, FD, and RST. The first plot illustrates a pre-charge phase in which the photodiode is reset with the TX gate being driven to Vtx1, and all of signal electrons are readout from the photodiode to FD. At this time, the PD bottom potential is set to Vpin which is determined by the completely depleted PD. The second plot illustrates that the TX gate turns off and the PD accumulates signal electrons until exposure time 1. The third plot illustrates that at the end of exposure time 1, a skimming pulse Vtx2 is applied to the TX gate, which is smaller than Vtx1, and a part of signal electrons are readout to FD. That is, the TX gate is driven just hard enough to skim off only part of the charge. At this time, the PD upper potential is set to Vskim which is the same as the channel potential beneath TX gate. As a result, the remaining signal is Vpin−Vskim. The fourth plot illustrates that at the beginning of exposure time 2 the TX gate turns off and the PD starts to accumulate signal electrons until the end of time 2. Thus, at the end of time 2 prior to readout, the charge has increased to Vex, as illustrated in the fifth plot. The sixth plot illustrates read out after time 2. At time 2, the TX gate turns on, and all of the signal electrons are readout to FD.
FIG. 1C is a readout timing diagram summarizing the pulse timing at the transistor TX at different times. The gate voltage Vtx1 is sufficient to turn transistor TX completely on but voltage Vtx2 only weakly turns transistor TX on during charge skimming. The different phases of operation of the unit pixel include a first exposure, charge skimming operation, and second exposure.
FIG. 1D illustrates how the conventional exposure time control method with charge skimming is used to extend the dynamic range. The transfer characteristic relates the output to the light intensity. The charge generated by the PD depends on the light intensity. For low light intensity conditions the transfer characteristic is linear with a slope “a” and charge skimming is not performed. When the charge skimming is performed, a smaller portion of the signal charge flows to FD. Therefore, the slope is changed from “a” to “a x Tex2/Tex”. This slope is smaller than “a” and as a consequence the dynamic range is expanded.
FIG. 1E illustrates some of the drawbacks of the conventional exposure time control method using charge skimming. For the example where the PD is a pinned photodiode, the bottom potential of the pinned photodiode is basically defined by the photodiode's n-type implant dosage. This bottom potential of photodiodes in an image pixel array has a distribution because of the fluctuation in LSI fabrication process. Additionally, the Vskim is different among each pixel due to the fluctuation of transfer gate (TX) threshold voltage. Each Vpin is thus different in each PD and each Vskim is different in each Tx, which results in a distribution of Vpin−Vskim. However, Vpin−Vskim is the remaining charge after skimming, which also determines the breakpoint of the transfer characteristics. These distributions in breakpoint cause fixed pattern noise in the image that degrades the image quality.
In contrast, for the case where the PD of the unit pixel is a common n-type photodiode, fixed pattern noise doesn't occur, but this type of photodiode has large leakage. As a result, the image quality is not good for low light level scenes.
Other HDR image sensor approaches that have been proposed also have drawbacks. One approach is to use photodiodes with different areas. In this approach, the image pixel array consists of two kinds of pixels with different photodiode areas. One photodiode area is a normal size, and the other is smaller. Pixels with the normal photodiode area are for low and normal light level scenes. Pixels with smaller photodiode area are for high light level scenes. The two kinds of signal are added in a digital signal processor (DSP) after compensation. As a result the dynamic range is expanded. However, one drawback of this approach is that the number of available pixels for low light level and high light level scenes is half the number of pixels in the array. This results in a degradation of the resolution if all pixels are used to create an image.
Still yet another proposed HDR image sensor utilizes a logarithmic amplifier. The sensor has a logarithmic amplifier coupled to each pixel instead of a linear amplifier. This logarithmic amplifier expands the dynamic range. But, this sensor has two major weak points. One is low sensitivity, and the other is fixed pattern noise from differences in amplitudes among the logarithmic amplifiers.
Therefore, what is desired is an improved HDR image sensor capable of 100 dB dynamic range without having other drawbacks, such as fixed pattern noise, large random noise, or degraded resolution.